1. Field of the Invention
The present invention relates to a semiconducting ceramic material, a process for producing the ceramic material and a thermistor comprising the ceramic material. More particularly, the present invention relates to a BaTiO3-type semiconducting ceramic material which exhibits the characteristic of positive temperature coefficient of resistance (PTC characteristics), to a process for producing the ceramic material and to a thermistor comprising the ceramic material.
2. Background Art
Conventionally, a BaTiO3-type semiconducting ceramic material is widely employed in the manufacture of PTC thermistors since the ceramic material exhibits the characteristic of positive temperature coefficient of resistance (PTC characteristics). The PTC thermistor is widely employed for the demagnetization of a cathode-ray tube or in a heater.
In order to reduce resistance, there has been keen demand for a PTC thermistor comprising a laminate which includes semiconducting ceramic materials and internal electrodes. Since a base metal such as Ni is employed to form an internal electrode of the PTC thermistor, the semiconducting ceramic material must be re-oxidized after the material is fired in a reducing atmosphere. The re-oxidation of the semiconducting ceramic material is carried out in order to obtain the PTC characteristics of the material through the re-oxidation of grain boundaries of the material.
However, it is difficult to completely re-oxidize the semiconducting ceramic material at a temperature sufficiently low to prevent oxidation of the internal electrode formed of a base metal.
In order to solve this problem, for example, Japanese Patent Application Laid-Open (kokai) No. 8-153604 discloses a process in which a material having a low firing temperature is employed as a semiconducting ceramic material. However, such a process is not necessarily satisfactory.